m68k: merge the mmu and non-mmu versions of sys_m68k.c

There is a lot of common code in the sys_m68k.c files. The mmu and non-mmu
versions can easily be merged into a single file.

There is really only 2 functions that differ in the 2 cases. A single
ifdef on CONFIG_MMU can take care of this. Alternatively we could break
those 2 functions out and maintain sys_m68k_no.c and sys_m68k_mm.c with
just this code in it (Makefile could then just build the right one).
Does anyone have strong feelings on which way they want this done?

Signed-off-by: Greg Ungerer <gerg@uclinux.org>